The present invention relates to the field of wireless communications, more particularly to a method and apparatus for estimating signal to noise power ratio in a multi sub-channel CDMA receiver. The estimation may be effected to control the transmitting power between base station and mobiles.
CDMA (Code Division Multiple Access) is a digital air interface standard, employing a commercial adaptation of military spread-spectrum single-sideband technology. More specifically, it is a form of digital cellular phone service that is a spread-spectrum technology which assigns a code to all speech bits, sends scrambled transmission of the encoded speech over the air, and reassembles the speech to its original format. In a CDMA system, each user is given a unique sequence (pseudo-random code) . This sequence identifies the user. For example, if user-A has sequence-a and user-B has sequence-b, a receiver wanting to listen to user-A would use sequence-a to decode the wanted intelligence It would then receive all the energy being transmitted by user-A and disregard the power transmitted by user-B. Because system users are isolated by code, they can share the same carrier frequency, partially eliminating the frequency reuse problem encountered by other cellular systems, such as AMPS and DAMPS.
CDMA is an interference limited system. Although it has a soft capacity limit, each user is a noise source on the shared channel and the noise contributed by users accumulates. This creates a practical limit to how many users a system will sustain. Mobiles that transmit excessive power increase interference to other mobiles. For CDMA, precise power control of mobiles is critical in maximizing the system""s capacity and increasing battery life of the mobiles. The goal is to keep each mobile at the absolute minimum power level that is necessary to ensure acceptable service quality. Ideally, the power received at the base station from each mobile should be equal, resulting in the minimum signal to interference. If the power of each user with a cell is not controlled such that they do not appear equal at the base station receiver, then the near-far problem occurs. This problem occurs when many mobile users share the same channel. In general, the strongest received mobile RF (Radio Frequency) signal will capture the demodulator at a base station In CDMA, stronger received signal levels raise the noise floor at the base station demodulators for the weaker signals, thereby decreasing the probability that weaker signals will be received.
Thus, in CDMA mobile systems, closed loop power control is essential to maintain the received power level at the base station or mobile station receivers. The receiver side should perform an estimation of the received signal and noise power level, and compare it with the Signal to Noise Ratio (SNR) threshold. If the received SNR level is lower than the threshold, a power-up command, typically a single bit, is sent to the transmitter to increase the transmitting power. Otherwise, a power-down command is sent to lower the transmitting power.
Such a power control command is embedded in the forward CDMA channel (used for transmitting from the base station or cell site) in the form of reverse power control bits. These occur in pseudo-random positions in each 1.25 ms interval (Power Control group or PWC), or 16 times per frame, as per the Air Interface Standard IS-95 and the CDMA2000 Standard. Each power control bit is interpreted as a command to raise or lower power by a predetermined increment. Each base station makes power control decisions independently for each mobile. The mobile station is responsible for demodulating the power control bits and raising or lowering its power accordingly. The goal of the power control is to maintain the reverse channel transmit power at the lowest possible level commensurate with adequate error performance.
One approach common to CDMA systems is the use of a Rake receiver for combining the information obtained from several resolvable multipath components in order to provide an enhanced signal with better voice/data quality. The Rake receiver includes a bank of correlators, also referred to as demodulators, each of which correlate to a particular multipath component of the desired signal. The correlator outputs, commonly referred to as the Rake channels or fingers, may be weighted according to their relative strengths and summed to obtain a final signal estimate.
Obviously, it is important to accurately obtain an estimation of the received signal to noise power ratio for proper power control bit generation. In the current implementation of the Air Interface Standard IS-95 reverse channel (used for receiving calls at the cell site from the mobiles), there is only one channel per user link. The reverse channel uses 64-ary orthogonal modulation symbols, specifically Walsh functions of order 64, and the SNR estimation is performed by processing the Walsh spectrum of the Rake output. There are 64 elements in the orthogonal demodulation output of the Rake receiver. By regarding the largest element as the signal contribution and the rest as being caused by noise and interference, one can obtain the SNR estimation at the Rake receiver output.
Unfortunately, difficulties in efficient SNR estimation arise with the most recent CDMA systems, implemented in accordance with the third generation 3G IS-95 Air Interface Standard, whereby each channel between one user and the base station can have up to four integrated sub-channels, with their rates and relative power levels changeable. In addition, these channels use Binary Phase Shift Keying (BPSK) modulation, a digital linear modulation technique, as opposed to multi-ary orthogonal modulation. In the 3G IS-95 wireless CDMA system, the pilot signal is used to perform coherent detection of the BPSX signal, and there is no longer the multi-ary output as in the IS-95 system.
Thus, there exists a need in the industry to provide a method and apparatus for performing signal to noise power ratio estimation in a multi sub-channel CDMA receiver.
In summary, the present invention provides a method and an apparatus for generating a control signal that conveys an estimation of the signal to noise power ratio (SNR) in a multi sub-channel CDMA channel. Such a channel can typically be the reverse channel between a mobile transmitter and a base station. The control signal can be used to regulate the power-up/power-down command sent by the base station to the mobile transmitter.
In a specific example, the apparatus is incorporated in a CDMA receiver. The estimated SNR conveyed by the control signal is compared to a dynamic threshold set in accordance with the Frame Error Rate (FER) measured at the output of the receiver. A power control command is then determined based on the comparison. More specifically, if the SNR is higher than the threshold, a power-down command is sent to the mobile transmitter. If the SNR is lower than the threshold, a power-up command is sent.
Under this example, the SNR estimation needed for closed loop power control is implemented at the Rake receiver combiner output. The combiner performs complex maximum ratio combining of the Rake finger outputs, each weighed by the complex conjugate of the channel estimation from the pilot channel. The imaginary part of the output is due to the noise and the estimation error. By averaging the squares of real and imaginary parts of the combiner outputs over one power control group and over all channels, the total received signal energy and noise power can be estimated. Since all the channels are considered, this novel method does not need to know the exact power allocations among all sub-channels, which might change due to adjustment or inaccuracy of the physical devices.